I never/no longer offer to do a free test for translation agencies - neither do I recommend doing it occasionally. The better your work is and the more it is worth, the less likely it is that you are getting a decent job if you're getting a paid job at all - that's my experience.
There are always other things to keep you busy when you're not getting paid for it - try getting direct clients (contact websites, businesses, publishing companies, and do some sample translation for them if that's possible, as with websites).
Doing free tests for translation agencies is time wasted as far as I am concerned - it doesn't pay and, secondly, even if it pays (mostly they'll try to pay less than for a real project), you won't get a decently paid follow-up job - that's also my experience. The feedback for your test translation from an agency is also often irrelevant = the agency itself has no idea about the quality of your work and if they have it checked by someone, that someone either works for much less than you (and their review is always questionable), or if they gave you a text that they already have a translation of, they will try to criticize yours for whatever arbitrary reason they can find, even if it's simply a word or term that you rightfully use but which is not used in their sample. The thing is that an agency will always try to find reasons to get someone for as cheap as possible (and many at absolutely unacceptable rates). That's how the majority of them work these days. If someone's happy getting caught in a spiral of low-paying jobs, they should go ahead. Don't tell me later I didn't warn you.
Recommending to a newcomer that test translation are generally a good thing is not something I can agree with. There are way to many clowns out there. And they're not funny.
[Edited at 2014-11-29 16:22 GMT]
The picture you paint may be accurate in many cases, but I don't think it's universal.
I recently did a test translation for a company that I had identified as specialising in the field I work in (renewable energy). They agreed to my rates (without quibbling) before sending the test through, and the feedback I received was very useful and well-considered (mostly praise and not at all aimed at knocking my rate down). The work was checked by the MD of the company (a full member of the ITI). All in all, the work involved in doing a short test piece felt like a worthwhile investment - and the company also invested their time in checking it thoroughly and providing good feedback. I don't do free tests all the time - I have plenty of work anyway - but I wouldn't rule it out for the right company.
There are indeed a lot of clowns out there, but there are better agencies too who are worth working with.
[Edited at 2014-11-29 17:04 GMT]